A system and method for color image enhancement is disclosed in U.S. Pat. No. 4,984,072 issued Jan. 8, 1991 and U.S. Pat. No. 5,093,717 issued Mar. 3, 1992, both of which are incorporated herein by reference. In these patents, a system and method is described for color enhancing monochrome images by way of a color transfer function which converts gray-scale information in a monochrome image to unique values of hue luminance and saturation. A motion picture is divided into frames from which certain key frames are selected. Each key frame contains objects, scenery, characters, etc. which are representative of those in the subsequent several frames. Accordingly, by selecting colors for different regions of a key frame, a color legend or template is established from which the subsequent frames can be colored, until a new key frame is reached. In practical terms, a key frame may represent a particular scene and thus will serve as a key frame until the next scene (possibly shot in a different location) takes place.
A key frame is colored by designating regions of the key frame which have a similar hue. For instance, flesh tones of the characters may constitute one region while certain pieces of clothing another region. Certain pieces of scenery such as a house or woods may constitute other regions. It may also turn out that seemingly unrelated parts of the key frame may have similar hues and thus be included in the same region.
Each region is selected using various tools which aid the key frame colorist. A free hand tool allows the key frame colorist to trace the edges of an area to define that area as a region. Another drawing tool allows the colorist to define polygons rather than freehand traced shapes. Still another tool allows the key frame colorist to select certain threshold values and then designate whether gray-scale values which are numerically above or below the threshold value are included in a region.
Once the regions are selected, it is time to select the actual colors which are to be used in the color transfer function for applying colors to the pixels in each region. Colors are defined using the Hue, Luminance, Saturation (HLS) System.
There is shown in FIG. 3 a diagram of the double cone representation of the visible HLS space 40. The actual HLS space is cylindrical. The volume outside of the double conic space, however, is either all white or all black, depending on which half of the cylinder you are in. For this reason, the double conic space is the only space of real concern. In this diagram, a point A is also shown. Point A consists of three components:
a hue value which is the angular displacement .alpha.; PA1 a saturation value which is the distance D measured from the central axis of the double conic space; and PA1 a luminance value which is measured against luminance axis 48.
Luminance axis 48 shows values from 0 to 255 corresponding to a 256 values (or 8-bit) representation.
These three HLS values which designate point A, undergo an HLS to RGB conversion in block 42 yielding a specific RGB value 46 stored in RGB value look up table 44. Look up table 44 also contains 256 values numbered 0 to 255. In the context of coloring a monochrome image, each RGB value in look up table 44 corresponds to a gray-scale value between 0 and 255 (or an 8-bit gray-scale value word). Accordingly, each region has its own look up table and each pixel gray-scale value in a particular region has an associated RGB value which is used in the final color image.
Colors are selected using a color wheel displayed on a video screen with a pointing device and/or a keyboard input. The color wheel displays a full spectrum of colors, with the actual number of colors being displayed being limited by the video capabilities of the equipment being used. The color wheel is a cross-sectional slice through the double conic space which defines the hue, luminance and saturation (HLS) color representation system. Once colors have been selected for each region, the three information files (pixel gray-scale information, region information and color information) can be used by a color transfer function to create a color image representative of the original gray-scale (monochrome) image.
In frames subsequent to the key frame, it is only necessary to select a region which has an assigned color look up previously selected during key frame coloring. This takes place until the next key frame at which time, the next key frame serves as the template or legend for the assignment of colors to particular regions. Because motion from frame to frame is typically not drastic or sudden, the entire region templates can be moved from frame to frame with only minor touch-up work necessary to sufficiently designate a region.